1. Field of the Invention
This invention relates to a trapped-vortex pair flowmeter which uses a fluctuating pressure field created by an oscillating pair of vortices to measure a very wide range of volumetric flow rates of any type of fluids.
2. Description of the Prior Art
Several devices exist for measuring fluid flow. Two major groups of flowmeters include those which have moving parts such as turbine flowmeters and those which have no moving parts. In an article entitled "Fluidic Flow Measurement and Control Devices" in Measurement and Control, Vol. V, No. 10, October 1972, R.F. Boucher and J.K. Royle describe many types of flowmeters. Flowmeters having moving parts rely on the frequency of a mechanical element to determine volumetric flow rates. Flowmeters having no moving parts rely not on the frequency of a mechanical element but rather on the frequency of a portion of the fluid which has oscillation characteristics due to hydrodynamic instability. Either a flow or pressure sensor detects such oscillation characteristics.
R.F. Boucher and J.K. Royle further teach types of flowmeters including vortex shedding flowmeters, vortex precession flowmeters and oscillating jet flowmeters. Existing oscillating jet flowmeters basically operate by having a turbulent jet flowing into an expansion chamber which is divided into two by a splitter thus forming two outlets for the flow. Further downstream within the chamber, both outlets of the expansion chamber merge into one outlet. Two nozzles are symmetrically situated at right angles to a supply nozzle, located upstream from the expansion chamber. Both symmetrically situated nozzles or control ports are connected together to form a control loop or feedback loop. A pressure difference across the jet varies with time and causes an alternating clockwise then counterclockwise flow in the control loop which forces the jet to alternate back and forth between both outlets within the expansion chamber. As the flow rate increases, the frequency of strong spontaneous oscillations also increases. Flow or pressure sensing devices provide measurement readings on the frequency of the oscillations which is proportional to the fluid flow rate.
R.F. Boucher and J.K. Royle further teach that "suitable body geometry is a matter of considerable investigations. Not only must the geometry be selected from within the limited range of parameters for which oscillation is guaranteed, but choice is further limited by jitter (random frequency variations), noise, reasonableness of geometry, low manufacturing tolerances on dimensions and of course, low characteristic and minimum Reynolds numbers." R.F. Boucher and J.K. Royle further teach that a feedback loop or control loop has fundamental importance in a flowmeter having no moving parts since the inertia of the feedback loop or control loop determines the time delay in establishing switching flow in each half cycle.
In an article entitled "Experimental Investigation of a Fluidic Volume Flowmeter" in Journal of Basic Engineering, March 1970) M.P. Wilson, Jr., C.H. Coogan, Jr. and K. Southall generally describe flowmeters, the effect of design parameters on oscillation frequency, and locations of feedback inlet nozzles.
Another type of flowmeter which has no moving parts is the Coanda Meter which operates with a feedback loop. In an article entitled "Gas Measurement, Domestic Gas Meter Adjustment Using Coanda Mastermeters", the Australian Gas Journal, June 1982, Dr. P.H. Wright describes Coanda meters which are essentially fluidic feedback oscillators based upon the Coanda effect. The Coanda effect is the phenomenon in which a turbulent fluid jet flows into a diverging channel and tends to follow only one of the diverging walls. Random fluctuations in the main fluid jet determine which wall the fluid jet follows. A downstream portion of the fluid flow is directed through a feedback channel or feedback loop back upstream to a low pressure region near the main nozzle where the fluid jet begins to bend toward one wall. The redirected flow through the feedback channel or feedback loop into the low pressure region causes the fluid jet to detach from one wall and flow along an opposite wall. In an article entitled "The Coanda Meter--A Fluidic Digital Gas Flow Meter", J. Phys. E: Sci. Instrum., Vol. XIII, 1980, printed in Great Britain, P.H. Wright explains the basic operation of a Coanda meter.
In an article entitled "Whither Metering", the Institution of Gas Engineers, 123rd Annual General Meeting, Princess Theatre, Torquay, Great Britain, May 13-15, 1986, at pgs. 25 and 26, a fluidic type meter and its principle of operation is generally described. The article states that since flow oscillations stop at a given flow rate in a fluidic-type meter, "It is unlikely that a single such meter will be able to cover the required flow range alone and a separate low flow sensor will be needed." The article further states that considerable efforts are being devoted to investigate alternative sensing devices which have capabilities to cover turbulent fluid flow oscillations according to the Coanda effect as well as the characteristics of low flows.
McLeod, U.S. Pat. No. 3,500,849 teaches a free-running oscillator having a closed fluid oscillator chamber, an inlet nozzle for directing a cntinuous power stream into one end of the oscillator chamber and a single fluid outlet port which is offset axially from the power stream. The '849 patent teaches a device only for oscillating a free-running fluid stream.
Burgess, U.S. Pat. No. 3,589,185 teaches a flowmeter having an obstacle assembly mounted within a flow conduit. Such obstacle assembly generates strong stabilized oscillations in the downstream wake of the flow conduit. The '185 patent further teaches the obstacle assembly as a contoured block having a triangular or delta-shaped cross section which is uniform throughout the longitudinal axis of the block. The '185 patent teaches a sensor which protrudes into a downstream portion of the fluid flow stream. The sensor may be in the form of thermistor, sound-responsive transducer, or a differential pressure sensor.
Adams, U.S. Pat. No. 3,640,133 teaches a flowmeter having a fluid interaction chamber and a feedback loop which provides oscillation to the flow. The '133 patent further teaches a flowmeter which sets up a frequency proportional to the volumetric flow rate. The proportional relationship remains the same for any compressible or incompressible fluid as long as the flow is turbulent and subsonic.
Tippetts et al, U.S. Pat. No. 3,690,171 teaches a fluidic oscillator having an entry nozzle, control ports or feedback loop, outlet channels and a splitter. The '171 patent further teaches a channel extending between the outlet channels which houses a microphone providing electromagnetic differential pressure sensing means electrically connected with a frequency meter calibrated in terms of flow rate. The relationship between frequency of oscillation and flow rate depends on the dimensions of the fluidic oscillator, thus physical dimensions of the fluid flow measurement device must be changed as various flow rates change.
Williamson, U.S. Pat. No. 3,885,434 teaches a flowmeter having one moving part, a ball inside of a tube. The '434 patent teaches a ball inside of a tube and a stop to prevent the ball from moving longitudinally within the tube and with the flow of the fluid. The ball which is detected by inspection from outside the tube has rotational and lateral movement proportional to the rate of the flow of the fluid.
Haefner et al, U.S. Pat. No. 4,085,615 teaches a linear flowmeter having an interaction chamber and a feedback loop which causes fluid oscillations. The frequency of oscillation is proportional to the volumetric flow rate through the linear flowmeter.
Bauer, U.S. Pat. No. 4,184,636 teaches a fluidic oscillator having a chamber with a common inflow and outflow opening into which a jet is issued in a generally radial direction. The '636 patent further teaches vortices which alternate in strength and position to direct outflow through the common opening along one side and then the other side of the inflowing jet. The concentration and distribution of a sweeping spray pattern can be readily controlled by properly configuring the oscillator and/or output chamber.
Bauer, U.S. Pat. No. 4,244,230 teaches a fluidic oscillator flowmeter having two members of semi-ovate cross section transposed transversely across a pipe with the major axis of the semi-oval parallel to the flow direction. Both semi-ovate members are slightly spaced apart to define a downstream tapering nozzle between the two semi-ovate members. The downstream ends of both semi-ovate members are formed as downstream-facing cups. The '230 patent further teaches a third body member having an oscillation chamber defined therein to receive flow from a nozzle. The oscillation chamber of the third member has a concave U shape portion into which a fluid jet is directed.
The '230 patent teaches a pair of tiny pressure ports defined in an impingement wall end of the oscillation chamber. The jet impingement point is on the far wall of the oscillation chamber. The '230 patent teaches a chamber which may be asymmetric. The exiting flow is completely blocked by one of the vortices during some phase in the oscillation. The '230 patent further teaches that the side walls of the oscillation chamber are concave with respect to the axis of the main fluid jet and the oscillation chamber has a closed bottom through which no fluid escapes.
Herzle, U.S. Pat. No. 4,550,614 teaches an oscillatory flowmeter having a diverter which acts to split flow from a power nozzle into a control stream that is diverted toward the inlet of an associated feedback loop and an output stream that is directed toward an output duct. The '614 patent further teaches the sensing of resultant fluidic forces exerted alternately on the diverter, the sensor outputs are processed to generate a sinusoidal wave from which volumetric flow or mass output signals are derived.
Okabayashi et al, U.S. Pat. No. 4,610,162 teaches a fluidic flow meter having a combination of an upstream fluidic element and a downstream fluidic element. The '162 patent further teaches a bypass passage disposed parallel to one fluidic element having a jet nozzle with a smaller opening area than the opening area of a jet nozzle of the other fluidic element. The bypass passage also has a valve means, preferably a diaphragm type governor valve, which has a main valve member and an auxiliary valve member adapted to open by a fluid pressure upstream thereof when the main valve member is in a closed position.